1. Field of the Invention
The invention relates to apparatus for a ground start circuit suited for use in telecommunication applications, illustratively in conjunction with a ground start line, and a method for use in the circuit for invoking ground start operation on such a line.
2. Description of the Prior Art
Telephone lines designed for use as business lines, such as for example those connected to private branch exchanges (PBXs) or private automatic branch exchanges (PABXs) (hereinafter collectively referred to as PBXs), frequently utilize a ground start protocol to obtain dial tone from a central office. Use of such a protocol advantageously prevents "head-on" or glare from occurring, i.e. when an exchange connects a caller to a telephone line that is already in use by another party and thereby, due to the shared status of the line, superimposes one call over another.
In essence, a telephone line consists of a balanced pair of wires, commonly referred to as tip and ring wires (or leads), that form a loop with a central office at one end and, in this case, a subscriber's PBX at the other. Whenever a telephone line that employs a ground start protocol (hereinafter referred to as a ground start line) is in an idle state and hence not carrying an active call, then the central office will not apply any voltage between the tip and ring wires that form this line. In essence, the central office will permit the potential of the tip lead to float with respect to earth ground. Now, in order to obtain dial tone on the ground start line, such as for example to place an outgoing call, the PBX must first detect that no loop current is flowing in the line (i.e. that the line is idle) and, if so, then momentarily unbalance the loop by applying an earth ground potential to the ring wire. This, in turn, causes a direct current (DC) to flow between the ring side of the loop and earth ground. Once equipment located at the central office and connected to a far end of this line senses the unbalanced condition, then this equipment applies battery voltage, typically 48 volts DC, between the tip and ring wires of this line. At essentially the same time or even somewhat before, the PBX connects a load between the tip and ring wires to load the line. After the ring wire has been grounded for a period of time, the ring ground is removed to eliminate the unbalance in the line. At this point, the central office applies dial tone across the line. The subscriber who is connected through the PBX to this line can now initiate an outgoing call.
Various PBXs that are currently on the market utilize "timed" ground start circuits that disadvantageously maintain an unbalanced condition throughout a timed interval. Specifically, these timed ground start circuits first establish a connection between earth ground and the ring side of the line (i.e. a "ground mark") and then, by relying on an internal timer, maintain the ground mark throughout a pre-determined interval of time with the expectation that equipment at a central office connected to that line will detect and respond to the timed ground mark. Unfortunately, if the pre-determined time interval is too short, as it frequently is, then equipment at the central office may not detect the timed ground mark and hence not provide loop current and dial tone to the line. As such, any such PBX that utilizes a timed ground start circuit which employs a short interval timed ground mark may provide unreliable operation with certain central offices. Now, if such PBXs were to be universal compatible, i.e. operate reliably with equipment that exists at substantially any central office, then the timed ground start circuit should maintain the timed ground mark for at least three seconds, which is the maximum interval defined by telephone company specifications. Unfortunately, use of such a long interval may disadvantageously impede the response of certain central offices thereby limiting the amount of traffic which that central office can simultaneously switch. Moreover, such a timed ground start circuit requires additional support circuitry to appropriately set and reset the timer. This additional circuitry disadvantageously increases the size, cost and complexity of the ground start circuit while simultaneously decreasing its reliability.
Furthermore, not only do PBXs need to contain circuitry that momentarily unbalances a ground start line but also test equipment that is situated anywhere on a ground start line or at a subscriber's location at an end of the line must also contain ground start circuitry.
Specifically, in the current deregulated telephone environment, the local telephone company provides a telephone line, local telephone service and access through its switching facilities to long distance vendors, but is generally not responsible for the wiring ("inside wiring") and telephone equipment existing inside a subscriber's premise. Therefore, it has become quite advantageous to connect test equipment to the telephone line at a known point of demarcation, typically a wiring block located on the outside of a subscriber's building, where the local telephone company's wiring terminates and hence its responsibility ends. Doing so allows the subscriber, interconnect company or local telephone company to test the line and thereby determine not only whether a fault exists in the line but also importantly whether that fault lies within the local telephone company portion of the line or within the subscriber's portion, i.e. inside wiring and customer premise equipment, such as a PBX, connected thereto. This test equipment may illustratively contain circuitry that tests the amplitude and frequency of dial tone provided by the central office and appearing on the line. An example of such remotely mounted test equipment but designed for use with loop start lines is described in our U.S. Pat. No. 4,679,224 (issued July 7, 1987 to D. Lynch et al). Now, in order for this test equipment to properly function in conjunction with a ground start line, the test equipment must contain a ground start circuit that provides a momentary current path from the ring side of a subscriber loop to earth ground in order to obtain dial tone.
Test equipment designed for mounting at a subscriber's location must be highly reliable. Telephone lines, once installed, frequently remain in place and in service for several years if not longer. A fault, which requires that the line be tested, can occur anytime during this period of time. Consequently, any test equipment designed to remain connected to a telephone line at a subscriber location must be able to accurately operate upon demand anytime over a period of time spanning at least several years. Since, business subscribers frequently have a large number of separate lines entering a business location, the test equipment should also consume as little space as possible.
Unfortunately, ground start circuits known in the art frequently utilize a large number of components typically including large valued capacitors and relays. As such, these circuits have tended to be quite complex, expensive and bulky. Because of the large component count and mechanical contacts used in the relays, these prior art circuits also have limited reliability. As such, the prior art ground start circuits, particularly those using relays, have been found to be generally unsuitable for use in those applications, particularly but not exclusively occurring in remotely mounted telephone line test equipment, where small space, low cost and high reliability are needed.
Thus, a need exists in the art for a ground start circuit that is universally compatible with equipment at substantially any central office and does not rely on use of a timed ground mark. Moreover, such a ground start circuit should be simple, solid state (i.e. uses no relays), relatively inexpensive and formed of a relatively small number of components. In addition, such a ground start circuit should also consume a small amount of space and exhibit a very high degree of reliability.